1. Field of the Invention
The present invention relates to a method and a device for protecting ROM data which is a built-in user program of the microprogram control unit (MCU) and which provides high security.
2. Description of the Prior Art
A conventional protective device for ROM data is shown in FIG. 1.
FIG. 1 shows that the conventional protective device for ROM data consists of a specific register 30 inside the MCU 1, a control pin 31 for reading data in ROM 32, which is a user program, out of chip, a port control circuit 32, internal bus 33, data bus 34 and AND gate 35.
In the above construction, the output signal from AND gate 35 converts to high state when a high signal is inputted to control pin 31 and is written to specific register 30 simultaneously, and then the high signal enables port control circuit 32. Therefore, the data stored in ROM 2 was outputted through internal bus 33, port control circuit 32 and data bus 34 in turn.
This construction of the conventional protective device for ROM data has a problem that the protection of a user program is difficult, since the setting of control pin 31 and register 30 inside the MCU 1 was made by the same signal, making it possible for the data stored in ROM 2 to be read easily outside.